Kitchen countertops are made of a variety of materials, including laminated surfaces on chipboard, natural stone, and man-made solid surfaces simulating marble or granite. These include polymethyl methacrylate filled with alumina trihydrate.
When the countertop is provided with a cutout, generally rectangular in shape, to drop in an appliance such as a cooking stove, there are potential problems with the corners of the cutout acting as stress risers which increase stress concentration. Such appliances are provided with flanges to prevent the appliance from falling through the cutout opening. Appliances which cycle through high or low temperatures, especially cooking ranges, create thermal cycling and cyclical expansion and contraction of the countertop. Depending on conditions, this can eventually lead to cracks forming in the countertop corners which then could propagate out into the countertop and cause failure.
To minimize the stress riser effect of the corners, a 90 degree radius is normally used instead of a sharp corner. A larger radius provides less stress riser effect. However, there are limits to the size of the radius due to clearance required for the appliance.
Also, in a typical installation the front and back edges of the countertop cutout are much narrower than the edges to either side, when one looks at the counter from the position of the cook.
If the radius for the corner is partially cut back into the narrow edges, that can result in lower strength rather than stress relief.